The present invention is directed to a new source of VUV radiation and atomic radicals for chemical vapor deposition and growth of thin films at lower substrate temperatures than those required using pure thermal methods and with less radiation damage than conventional plasma means. The in-situ vacuum ultraviolet (VUV) lamp of the present invention can be located directly inside a processing chamber for use in photo-dissociating feedstock molecules and providing external energy to assist surface reactions. The in-situ VUV lamp, excited by the ring-shaped cold cathode of the present invention, can also provide a source of excited and ground state neutral atoms that may participate in sensitized atom-molecule dissociation reactions. The feedstock reactant molecules dissociated by VUV radiation and by sensitized reactions have products used for deposition, growth, etching, doping or polymerization of microelectronic films.
This in-situ source of both VUV photons and atomic species with large open area is made possible by a ring-shaped cold cathode geometry, without a center anode, which permits formation of a disc-shaped trapped electron beam. The disc-shaped trapped electron beam created plasma contains high energy electrons which are trapped within the ring-shaped cold cathode by electrostatic reflection. The spatial width of this electron beam created plasma may be kept below several millimeters, thereby allowing for much reduced radiation trapping of resonant radiation emitted since the absorption length of a resonant photon is less than or comparable to the width of the electron beam created plasma rather than greater than the plasma width as in conventional lamps. The high electron energy plasma efficiently generates both VUV radiation and atomic species, in a low pressure molecular gas such as deuterium, oxygen, hydrogen or nitrogen. The VUV radiation emitted from this disc-shaped trapped electron beam plasma occurs over a large area, typically 5 to 20 centimeters in diameter. No VUV windows are required for this lamp design. Radiation trapping effects, if present at all, will result in a diffuse volume of self-absorbed VUV radiation surrounding the disc-shaped plasma region. The VUV radiation will be used for photon assisted processing of microelectronic, photovoltaic, optical, and electro-optic films. A number of auxiliary grid electrodes are combined with the ring-shaped cold cathode to inhibit charged particles generated within the disc-shaped electron beam plasma from bombarding the processed sample when surface damage is undesirable. Alternatively, energetic electrons generated within the disc-shaped electron beam plasma can be purposely steered toward the processed substrate, thereby heating the surface, to enhance surface mobility of adsorbed species and thereby increase deposited or grown film density and reduce the number of pinholes in the film.
There is increased interest in low temperature and low radiation damage fabrication methods for use in III-V, II-VI and Si semiconductor processing, as well as for coating temperature sensitive materials such as heavy metal fluoride glasses. Vacuum ultraviolet photon assisted chemical vapor deposition (VUV-CVD) is one promising low temperature process used successfully to deposit metals, insulators and semiconducting films. The photon energies associated with commercially available laser and lamp photon sources, 4 to 7 eV, are in some cases inadquate for directly dissociating some feedstock reactant gases. The photon absorption cross sections of many polyatomic reactant gases used in CVD processes have peak photon absorption values in the vacuum ultraviolet (VUV) region beyond the reach of commercial photon sources. Photo deposition of hydrogenated amorphous silicon, for example, requires a VUV light source when using monosilane as a feedstock gas since monosilane does not absorb photons with wavelengths above 190 nm. The present invention of a cold cathode electron gun excited lamp provides an in-situ large area vacuum ultraviolet light source as well as a source of ground and excited atomic gas species. Illustrative gases well suited to operation with cold cathode electron guns are atomic helium, molecular hydrogen, molecular oxygen, and molecular nitrogen. Both VUV photons and atomic radicals are available in electron beam excited He, N.sub.2, O.sub.2 or H.sub.2 plasmas and may be used to assist low temperature chemical vapor deposition, the growth of thin films on substrates, and the passivation of crystalline defects in epitaxial films or bulk wafers. The hydrogenation of previously deposited polysilicon for use in thin film transistors is a specific example of the use of VUV photons and atomic radicals. For example, see Nakazawa et al., Applied Physics Letters vol. 51 pp. 1623-1625 (1987).
A ring shaped cold cathode creates beam electrons that are injected into the ambient gas to sustain a spatially confined disc-shaped electron beam plasma. The VUV photons, ground state atoms and excited gas species generated by the disc-shaped electron beam plasma, can initiate and sustain both gas phase homogeneous and surface induced heterogeneous chemical reactions. In the present invention control grids and purging gas jets serve to enhance the performance of the VUV and atomic radical source, especially for applications over a much wider area and when using hydrogen, nitrogen, oxygen, and helium feedstock gases. Such applications include polymer processing, film deposition, epitaxial film growth, and defect passivation of hetero-epitaxial films.
Photo-exposure, decomposition, and cross-linking of polymer materials all require an ultraviolet or vacuum ultraviolet light source of wide area to activate photochemical reactions in the polymer film. First layer resist hardening may, for example, be accomplished for use in multi-layer resist processing, thereby eliminating undesired reflow that occurs in unhardened layers when exposed to elevated temperatures. Photo-assisted chemical vapor deposition, photo-assisted etching, and photo-assisted growth of organic or inorganic based microelectronic, photovoltaic, and electro-optic films may also require a wide area VUV light source to dissociate feedstock reactant gases and thereby promote both rapid and low temperature processing. Compared to plasma processing, photon processing achieves the same reduction in the process temperature via input of external energy but with substantially less radiation damage since no electric fields are applied to the substrate.
There is a strong need for UV and VUV light sources of wide area in all of these photo-assisted process applications. Of special interest are wide area VUV lamps providing both a source of photons as well as a source of ground state and excited atomic species that are swept out of the active plasma and sent toward the substrate. The excited and radical species, created in the electron beam plasma, diffuse across a boundary layer to interact with the substrate. Sensitized atom-film reactions and VUV impingement on the film both assist heterogeneous surface reactions. There is no need for any external fields on the substrate being processed. All such fields are localized to the cold cathode electron gun excited lamp. The vacuum ultraviolet photons and gas sensitized reactions can also cause homogeneous reactant dissociation via volume photo-absorption and sensitized gas collisions, respectively. In the case of film deposition, nucleation occurs at selective adsorption sites on the substrate, leading to the formation of islands and finally, as growth proceeds through coalescence, a uniform film is created. VUV photons and sensitized excited atom collisions occurring at the surface will aid initial and intermediate stages of film deposition or growth, allow endothermic chemical reactions to occur, and increase surface mobility at lower substrate temperatures than possible with conventional pure thermal methods. The above low temperature growth or deposition also occurs with less radiation damage than that from conventional plasma assisted methods.
The in-situ VUV lamp of the present invention, when working with molecular D.sub.2, H.sub.2, O.sub.2, or N.sub.2 gas, also acts as a dissociation source for producing atomic deuterium, hydrogen, oxygen or nitrogen. The numerous ground state atomic hydrogen, oxygen, or nitrogen atoms that diffuse far (1-10 centimeters) from the confined disc-shaped beam of electrons can subsequently be pumped up to excited neutral levels following absorption of resonance radiation emitted from the plasma disc. VUV optical pumping of atomic ground state atoms raises their internal energy and allows them to participate in sensitized atom-molecule collisions which dissociate the reactant organic or inorganic polyatomic feedstock gas molecules either in the plasma volume or at the substrate surface. The present in-situ VUV lamp will run in D.sub.2, H.sub.2, N.sub.2, O.sub.2, and helium background gas pressures from 0.03 to 30 Torr as well as in mixtures of these background gases.
Examples of possible types of film deposition with the new VUV photon and free radical assisted deposition technique include: semiconductors, such as doped and undoped amorphous silicon, polycrystalline or epitaxial silicon; III-V materials such as gallium arsenide or InGaAs; insulators, such as silicon dioxide, aluminum nitride, silicon nitride, gallium nitride; crystalline diamond or diamond-like carbon films; and metallic conductors such as tungsten, aluminum, Al-Si or metallic silicides. Each deposited film's constituent atoms are introduced into the vapor phase by use of a specific feedstock reactant gas. For example, the gas molecules used in the deposition of SiO.sub.2 and Si.sub.3 N.sub.4 films include silane as a silicon donor and N.sub.2 O as an oxygen donor or ammonia as a nitrogen donor, respectively.
An additional application of this VUV and free radical assisted processing is an enhanced low temperature growth, as opposed to deposition, of native films of various materials. Illustrative examples include Si.sub.3 N.sub.4, SiC and SiO.sub.2 grown on Si. In this case, a feedstock gas compound reacts with the silicon substrate, assisted by VUV radiation and energetic free radicals provided by the new wide area source to assist formation of the desired silicon compounds at lower substrate temperatures than is possible without the external VUV and free radical energy. Again, this is accomplished with reduced radiation damage from ions or electrons that would be found in conventional plasma processing. A specific example of a native film growth is silicon dioxide (SiO.sub.2) on bare silicon wafers assisted by the absorption of VUV radiation by the oxygen containing feedstock gases and the creation of excited molecular and atomic oxygen species which diffuse into the growing SiO.sub.2 layer and oxidize the silicon in the substrate. Low temperature nitridation of SiO.sub.2 films, such as gate oxides, may also be accomplished by creation of excited nitrogen species to form a thin silicon nitride or oxynitride layer with minimum radiation damage.
Thin crystalline III-V layers can be grown directly on Si substrates but the control of heteroepitaxial conditions is still problematic especially for minority carrier devices. The two semiconductor materials have a physical mismatch between the lattice spacings, polar versus non-polar interface characteristics, and disparate temperature coefficients of expansion, all of which limit crystal quality via bulk crystalline defects and the presence of undesired surface states. Low temperature (300-400 degrees C.) atomic hydrogen assisted III-V film depositions will achieve lower defect material GaAs on Si than that obtained by thermal MOCVD. This occurs as the result of both lower deposition temperature and the beneficial effects of hydrogen on the different thermal expansion coefficients.
Prior work on the role of atomic hydrogen in MOCVD is varied. Much of this work emphasized post-deposition hydrogenation. However, the role of atomic hydrogen exchange from hydrides to organometallic molecules in decomposition reactions and formation of stable hydrocarbons has been recently studied by Mashita (J. Crystal Growth vol. 77, pp. 194 1986). Atomic hydrogen also aids donor and EL2 defect passivation following the work of Deutramont-Smith (Appl. Phys. Letters vol. 49, p. 1098 1986).
Initial exposure of the silicon substrate to arsenic containing gas, when done on a reconstructed surface, passivates the Si surface to avoid growth of undesired anti-phase boundaries. Indeed, much of the present success of GaAs heteroepitaxy on Si may be attributed to initial nucleation of the surface with arsenic. Alternatively, gallium passivated surfaces may be the most appropriate due to the high sublimation (gasification) rate in atomic hydrogen environments of arsenic. When the silicon surface is nucleated with only one element of the III-V compound and continued growth of the GaAs ensues, a highly charged interface results. Intense fields are created by the excess charge associated with the Ga-Si, Ga-As, and the As-Si bonds at the interface. This electrical field causes rearrangement of the interface material or compensation of the charge occurs through undesired defect generation. It has been shown that with the initial nucleation with gallium, the arsenic diffuses through this first layer to bond with the silicon. This arrangement, as reported by Kreomer (J. Crystal Growth vol. 81, p. 193 1987) may also be the reason for the diffusion of the Si into the GaAs thin film.
Arsenic passivation via dangling bond satisfaction with atomic hydrogen is achieved in order to reduce the interdiffusion of the Si into the GaAs by reducing the intense electrical fields that are created at the GaAs to Si interface in conventional processing. The specific process methodology uses excess atomic hydrogen and only gallium feedstock gas. Gallium nucleation on Si at the early stage of GaAs growth causes an interfacial excess negative charge created by the Ga-Si bond. Arsenic passivation via dangling bond satisfaction with atomic hydrogen may also reduce the interdiffusion of the Si into the GaAs by reducing the intense electrical fields that are created. In the early stages of growth, the atomic hydrogen species so created by the disc plasma will be used for passivation of the charge created at the III-V/Si interface by the polar on non-polar deposition. Hydrogenation provides for the passivation of the excess electrons that are formed in the creation of these bonds. Atomic hydrogen charge compensation of the interface could be done without degradation of the initial gallium or arsenic layer. The above process requires the unique VUV open structure lamp taught in accordance with the present invention.
The "windowless" lamps of the present invention with grids and purging jets have three major advantages in VUV-MOCVD or sensitized reaction MOCVD processes: (1) they provide a large area VUV photon source but they do not require VUV windows, rather a transparent metal mesh or an open disc may be used; (2) they allow for unimpeded flow of atomic species, created following electron impact dissociation of molecules in the plasma into the reaction chamber which can initiate a sensitized atom-molecule reaction, that is, atomic hydrogen is used for dissociating metal-organic molecules; (3) atomic hydrogen may be used for passivating GaAs-Si interfacial charges and for reducing residual carbon in the deposited film. All reactant gases are introduced at low partial pressures compared to the background hydrogen so that sputtering of the ring-shaped cold cathode is minimized. Inert gas or hydrogen purging in the disc-shaped electron beam plasma volume or near the ring-shaped cold cathode surface is also helpful for increasing operation stability of the disc-shaped electron beam plasma via in-situ discharge cleaning of the cathode surface.
Conventional VUV light sources include both high pressure and low pressure gas discharge lamps. These lamps are excited by either a radio frequency, microwave, or D.C. electrical source. A VUV transmitting window, such as MgF.sub.2, LiF.sub.2, sapphire, or CaF.sub.2 is used in these lamps to transmit the VUV resonance light generated in the enclosed plasma. The prior listed window materials are hydroscopic, their VUV transmission degrades the time and VUV exposure, and the windows are expensive. Conventional VUV light sources also include hollow cathode lamps with or without VUV windows. Such hollow cathode lamps are usually cylindrical rather than disc-like lamps and have a small circular area (&lt;3 cm.sup.2) with an optically thick plasma that traps resonance radiation. Finally, the hollow cathode discharge is usually a closed wall structure that substantially prevents outdiffusion toward the substrates of atomic species and free radicals created in the plasma region.
The need for a VUV window having a high optical transmission characteristic, the additional need for a source of excited atomic species, and the creation of wide area illumination of disc-like geometry in the VUV spectral region are all major problems not addressed in the conventional VUV lamp art. Conventional lamps then employ both VUV windows and enclosure walls to maintain the specific lamp gases at the required pressure within the lamp. These features inhibit or prevent the diffusion of atomic or molecular free radicals from the localized plasma region into the microelectronic electrooptic processing chamber, especially over a wide area. VUV lamps having diameters equal to or greater than 2-5 centimeters are simply not possible using previous gas discharge methods described above. Hence, they can directly illuminate only a portion of a 5-20 centimeter diameter substrate used in silicon or III-V microelectronics manufacture. This wide area processing requirement is greater for large area solar cells or flat panel displays since geometries often exceed 1000 cm.sup.2 in area.
Conventional enclosed VUV lamps using D.sub.2 or H.sub.2, for example, as a gaseous medium only create VUV radiation. They do not create and allow atomic hydrogen or atomic deuterium in ground or excited states to diffuse unimpeded from the lamp plasma into the region surrounding. Coventional lamps have walls and windows that are not open in structure as is the case for the ring-shaped cold cathode structure taught in accordance with the present invention. Hence, diffusion of atomic species from the plasma cannot occur over a wide area, if at all, for closed wall conventional VUV lamps. As emphasized in the process of the present invention, these atomic species can be used very efficiently both in sensitized atom-molecule reactions to cause dissociation of the feedstock reactant gases or to assist surface chemical reactions on substrates by providing a source of external energy.
The use of significant amounts of atomic hydrogen is known to facilitate the growth of crystalline diamond via both gas-solid heterogenous chemistry as well as by gas phase homogenous chemistry. What defines a significant amount of atomic hydrogen has not been adequately defined in the literature. However, the need of atomic hydrogen is well known. Reactant gases used in carbon and diamond film deposition processes may comprise various hydrocarbons such as CH.sub.4, C.sub.2 H.sub.6, MMA, etc. The VUV radiation from D (121.5 nm), H (121.6 nm), 0 (130 nm), He.sup.+ (122 nm) and He (58 nm) is able to directly photo-dissociate polyatomic feedstock molecules, such as CH.sub.4 and C.sub.2 H.sub.6 which absorb strongly only in the VUV region and absorb very little in the ultraviolet region and are used to deposit crystalline diamond or diamond-like films. It is noteworthy that using the in-situ VUV lamp of the present invention as compared to conventional lamps for the deposition of diamond-like carbon films is unique because deep VUV photons (&lt;75 nm), atomic hydrogen, and energetic electrons are all provided by this in-situ VUV lamp with external grids. The grids permit better independent control of relative amounts of energetic electrons impinging on the substrate.